As an important mass spectrometry technique, the Proton Transfer Reaction Mass Spectrometry (PTR-MS) is generally used for gas analysis. Specifically, at first gas sample molecules to be analyzed and pre-prepared reagent ions (generally, H3O+) are subjected to a proton transfer reaction to obtain sample ions, and the sample ions are analyzed by a mass spectrometer. The proton transfer reaction is carried out within a drift tube having a length of 10-30 cm and an air pressure of 1-2 torr, and a DC electric field is applied across the drift tube to drive the generated sample ions to the next stage. The time for the proton transfer reaction is determined by the value of E/N in the drift tube, wherein E is the intensity of the DC electric field, and N is the number density of the gas molecules. Once the reaction time is determined, the concentration of the sample ions can be calculated according to the constant of the reaction rate of the sample and the reagent ions, the intensity of the reagent ions and the sample ions measured by the mass spectrometer and the instrument parameters. Therefore, in the PTR-MS, the concentration of the sample gas can be accurately measured in real time without external calibration or internal calibration theoretically.
In the PTR-MS, the value of the E/N in the drift tube is very important to the quality of the mass spectrum. If the value of the E/N is too small, the reagent ions H3O+ are likely to form cluster ions with water molecules, leading to a complexed mass spectrum. If the value of E/N is too large, then too high ion energy will be caused although the formation of clusters is inhibited, and hence it is likely to generate fragment ions or trigger other reactions to generate interfering ions, which introduces interference in the spectrum. Typically, an appropriate value of E/N can be 120 Td, where 1 Td is about 10−17 V·cm2.
It should be noted that, since the DC electric field in the drift tube cannot confine the sample ions in the radial direction, the ions will be freely diffused in the radial direction when passing through the drift tube. When the ions are transmitted to the succeeding stage through a pore used for flow limiting in vacuum, about 90% of the ions will be lost. As a result, the sensitivity is reduced.
In order to improve the transmission efficiency, recently, some researchers have introduced the ion funnel technology into the PTR-MS, for example, as described in the document “Anal. Chem. 2012, 84, 5387-5391” or “Int. J. Mass Spectrom. 414 (2017) 31-38”. The ion funnel to which an RF voltage is applied can effectively focus ions, and thus realize above 10× ion transmission efficiency. However, an RF electric field capable of trapping ions effectively will dramatically heat the ions. Consequently, the value of E/N is too large and too many fragment ions will thus be generated, thereby seriously interfering the spectrum. In addition, the RF electric field is characterized by mass selection or discrimination against ions, and thus it is not ideal for many situations.
On the other hand, in the U.S. Pat. No. 6,639,213 and the document “Int. J. Mass Spectrom. 301 (2011) 166-173”, an ion drift tube for analyzing the ion mobility has been proposed, wherein, by a small radius of the drift tube and a proper ratio of radius/spacing, a DC electric field can be used to periodically focus ions, and a high transmission efficiency can be realized without significantly reducing the resolution of the ion mobility spectrum. Conceivably, if a similar drift tube is applied to the proton transfer reaction mass spectrometer, a high transmission efficiency can be achieved without additional ion heating. However, actually, this periodic focusing device can only ensure that the size of an incident ion pack will not be enlarged overly due to diffusion, but it cannot further compress the ion beam having a large incident cross-section. This device is actually a one-dimensional ion transmission device which does not have a real ion focusing or ion compression function. The drift tube in the proton transfer reaction mass spectrometry usually requires a large reaction region to ensure its sensitivity. Hence this device is not applicable.
Therefore, it is necessary to provide a proton transfer reaction mass spectrometer which can ensure a high transmission efficiency of high-throughput sample ions in a drift tube and meanwhile will not generate fragment ions so that the spectrum quality is ensured.